Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Fixed maturity securities available for sale, amortized cost	$ 8,739.7	$ 8,870.8
Equity securities available for sale, cost	$ 367.9	$ 381.8
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	800,000,000	800,000,000
Common stock, shares outstanding (in shares)	55,029,813	55,941,480
Treasury stock, at cost (in shares)	95,124,719	94,041,583